In recent years, the problem of rehospitalization has come under intense focus as a major contributor to preventable morbidity and escalating healthcare costs. Substance use disorders are strongly associated with poor health outcomes and highly inefficient use of healthcare services, including repeat hospitalizations. Interventions that increase adherence to recommendations for outpatient medical care and substance abuse treatment could potentially help recently-hospitalized individuals with substance use disorders to avoid unnecessary rehospitalization, associated morbidity, and medical expenses. The current study is a randomized controlled trial comparing the effectiveness of Navigation Services to Avoid Rehospitalization (NavSTAR) vs. Treatment-as-Usual (TAU) for hospital patients with co-occurring medical problems and substance use disorders. Applying Andersen's theoretical model of health service utilization, NavSTAR will employ the promising strategies of Patient Navigation and motivational interventions to facilitate engagement in outpatient medical and substance abuse treatment, thereby lowering the likelihood of rehospitalization. Patient Navigators embedded within the substance abuse consultation liaison service at a large urban hospital will deliver patient- centered, proactive navigation and motivational services initiated during the hospital stay and continued for 3 months post-discharge. Participants randomized to TAU will receive usual care from the hospital and the substance abuse consultation liaison service, which includes referral to substance abuse treatment but no continued contact post-hospital discharge. Participants will be assessed at study entry and again at 3-, 6-, and 12-months follow-up on various measures of healthcare utilization, substance use, and functioning. The primary outcome of interest is time-to-rehospitalization through 12 months. In addition, a range of secondary outcomes spanning the medical and substance abuse service areas will be assessed. The study will include an economic evaluation of the cost, incremental cost-effectiveness, and cost-benefits of NavSTAR from the service provider perspective. This study addresses a problem of national significance. The extent of avoidable rehospitalizations in the U.S., and among individuals with substance use disorders specifically, is indicative of public health risks and constitutes a key driver of healthcare expenditures. This study is innovative in its application of Patient Navigation and motivational intervention strategies with the specific purpose of avoiding rehospitalization among individuals with co-occurring medical problems and substance use disorders. If NavSTAR proves to be effective and cost-effective in reducing rehospitalizations in this high risk patient group, it would have important implications for designing hospital discharge planning services, informing national cost containment initiatives, and improving public health.